Not Applicable
1. Field of the Invention
This invention pertains generally to phased antenna arrays, and more particularly to techniques for loading a associated transmission line to create a phased array.
2. Description of the Background Art
Phased array antennas are used in a number of applications, including both terrestrial and airborne radar, and satellite and mobile communications, where fast beam scanning is required or where mechanical rotation of the antenna is not practical and/or desirable. Antenna arrays typically comprise several radiating elements, each having a dedicated phase shifter. By adjusting the phase shifts for each of the individual radiators, it is possible to control the direction of the composite main beam.
There are several benefits to phase antenna arrays. For example, since the phase shifters are usually controlled electronically, the direction of the antenna main beam can be scanned very rapidly in comparison with a mechanically rotated antenna. In addition, a phased array requires no moving parts and can be constructed as a planar or conformal structure. A disadvantage of using a phased array, however, is that that each radiating element requires its own phase shifter, and high performance phase shifters that are currently available are expensive. In fact, the cost of the phase shifters can be as high as 40% of the total cost of the phased array.
Phased array antenna systems typically employ ferrite phase shifters or semiconductor device phase shifters. Ferrite phase shifters are typically difficult to manufacture, however, and hence tend to be expensive. Another disadvantage to the use of ferrite phase shifters is that they are exceedingly slow to respond to control signals, thus making them unsuitable for use in applications requiring rapid beam scanning. On the other hand, faster response can be achieved by utilizing semiconductor device phase shifters, but semiconductor phase shifters tend to suffer from high losses at microwave and millimeter wave frequencies, and have limited power-handling capability.
In an effort to overcome the aforementioned problems, fully distributed phase shifter circuits using Barium Strontium Titanate (BST), which is a ferroelectric material, have been investigated. In these circuits, the BST material is used to fabricate the entire microwave substrate on which the conductors are deposited in the form of thick films or bulk crystals, or to fabricate a portion of the substrate in the form of thin films sandwiched between substrate and conductors. These circuits rely on the principle that, since part or all of the microwave fields pass through the ferroelectric layer, the phase velocity of the waves propagating on these structures can be altered by changing the permittivity of the ferroelectric layer. This approach has several limitations, however, including: (1) the amount of capacitive loading due to the ferroelectric film cannot be easily varied to optimize phase shifter performance; (2) conductor losses are high in this structure due to the high dielectric constant of the ferroelectric film on which the transmission lines are fabricated; (3) the tunability of the film is not efficiently utilized; and (4) the control voltages required for this approach tend to be very high.
Therefore, a need exists for a phase shifting transmission line technology that addresses the aforementioned problems. The present invention satisfies that need, as well as others, and overcomes the deficiencies found in current techniques.
The present invention addresses the need for a new technology capable of loading transmission lines with BST capacitors, and is a significant departure from the continuous fully distributed techniques outlined above. In general terms, the invention comprises a low cost phase shifter circuit intended for use in phased array systems of the type described above. The invention further comprises a method of deploying BST capacitors to load the transmission line which addresses the shortcomings of present solutions. According to one aspect of the invention, lumped-element inductors are periodically connected in series as part of the phase-shifting transmission line. In accordance with another aspect of the invention, a program is described for determining circuit configuration and parameters for phase shifters according to the present invention.
By way of example, and not of limitation, a phase shifter according to the present invention employs thin film BST capacitors for periodically loading the transmission line as opposed to conventional continuous loading. The BST thin films can be deposited using RF sputtering, which is less expensive in comparison with semiconductor epitaxy. Moreover, the BST thin film phase shifter circuits may be manufactured using the high volume, low cost monolithic fabrication techniques developed by the IC industry. These aspects of the invention allow the creation of phase shifters at a fraction of the cost of currently available ferrite/semiconductor phase shifters.
Characterization of RF sputter deposited BST films at microwave frequencies has confirmed that it is possible to make phase shifters with extremely low losses at microwave/millimeter wave frequencies. In addition, these circuits are characterized by low drive power requirements, fast switching speeds, and high power handling capability. The aforementioned benefits make this technology extremely attractive for the manufacture of phased array antennas. Availability of the low cost phase shifters according to the present invention are expected to drive down the cost of phased array antennas and increase their acceptance in both military and civilian applications.
When designed according to the present invention, a phase shifter structure so behaves as a synthetic transmission line whose phase velocity can be controlled by properly configuring the values of external loading capacitors. This topology utilizes the tunability of the BST film effectively, thus requiring reduced control voltage levels. The use of periodic loading along the transmission line allows the structure to be optimized for improved loss performance. Furthermore, since the transmission lines are fabricated on low dielectric constant substrates, the conductor losses are reduced.
An object of the invention is to provide a low cost phase shifter circuit for use in phased array systems.
Another object of the invention is to use BST capacitors for periodically loading transmission lines.
Another object of the invention is to provide phase-shifter transmission line inductance by utilizing lumped-element inductors periodically placed in series along the transmission line.
Another object of the invention is to provide BST thin film phase shifter circuits that are inexpensive and which can be manufactured using the high volume, low cost monolithic fabrication techniques developed by the IC industry.
Another object of the invention is to provide phase shifters with extremely low losses at microwave/millimeter wave frequencies.
Another object of the invention is to provide phase shifters that exhibit low drive power requirements, fast switching speeds, and high power handling capability.
Another object of the invention is to provide a phase shifter structure that behaves as a synthetic transmission line whose phase velocity can be controlled by changing the value of the external loading capacitors.
Another object of the invention is to provide a tunable phase shifter that requires low control voltages.
Another object of the invention is to provide transmission lines that are fabricated on low dielectric constant substrates and which have low conductor losses.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.